Gas valves are designed to be installed in liquid flow systems such as, for example, water main distribution lines or sewage collection systems, or liquid tanks, and are intended to discharge air (typically in water supply systems) or other gasses (e.g. in sewage systems), thus avoiding the formation and accumulation of gas pockets and bubbles which interfere with the liquid flow and which can also damage accessories and components fitted to the liquid system, and are designed to allow ambient atmospheric air into the system so to avoid the risk of contamination and possible pipeline and tanks collapse under internal sub-atmospheric pressure, when the system is drained from fluid.
A requirement for such gas valves is their ability to discharge effectively both large and small quantities of gas from a liquid system whilst, at the same time, being or becoming sealed against liquid discharge from the system. Conventional air purge valves are formed with a gas discharge outlet through which the gasses are discharged but which become sealed against liquid discharge by a float located in a valve housing and which becomes pressed against the outlet so as to seal it with a rising level of liquid in the valve housing.
Yet another requirement of a gas valve of the concerned type is to facilitate ambient gas ingress into the liquid system. This may be required for example, upon rapid discharge of the liquid system (e.g. at the event of deliberate draining or burst of a pie line, etc.). Thus, such valves should be suited for admitting ingress of ambient air at atmospheric pressure into the system at the occurrence of negative pressure (vacuum) within the liquid system. However, upon pressure re-built within the system, i.e. upon refilling, the inlet valve should reseal to prevent liquid egress from the system